Nigar, Farah
(2023)
Development of calcium phosphates from waste resources and fabrication of calcium phosphate-chitosan composites.
PhD thesis, University of Nottingham.
Abstract
Biowastes from agriculture, sewage, household wastes and industries comprise promising resources to produce biomaterials whilst reducing environmental impact. This study focused on utilising waste-derived materials (i.e., eggshells as calcium source, struvite as phosphate source and CH3COOH as dissolution media) to produce value-added products (i.e., calcium phosphate (CaP) derived biomaterials) using three different routes such as continuous flow hydrothermal synthesis, ball milling followed by sintering and flame spheroidisaion. Fabrication of composites (scaffolds and films) using commercial chitosan (Ch) and CaPs prepared using eggshells and struvite via three different methods mentioned above was also explored. The prepared materials were characterised via XRD, TGA/DSC, FEG-SEM, EDX, FTIR and TEM analysis.
Nanocrystalline hydroxyapatite (HA) formation was observed by reacting eggshell solution (prepared using HNO3) and diammonium hydrogen phosphate solution whereas magnesium whitlockite (Mg-WH - the second most abundant bone mineral), and HA were produced as a single phase or biphasic CaPs by reacting struvite with either calcium nitrate tetrahydrate or eggshell solution at 200 °C and 350 °C via continuous flow hydrothermal synthesis, where waste-derived precursors solutions were prepared using either HNO3 or CH3COOH. Structural characterisation and compositional analysis confirmed the sheet-like morphology of nano HA formation (with crystallite size ranges between 29 - 81 nm) at 200 °C. However, HA tubes (32-212 nm inner diameter and 15-49 nm wall thickness) and rod morphologies (32 – 86 nm) were observed at 350 °C using NH4OH and NaOH as pH buffer, respectively. Rhombohedral-shaped Mg-WH (23 – 720 nm) along with tube (50 – 290 nm in diameter with 20 – 71 nm wall thickness) and/or elliptical morphologies of HA (273 – 522 nm in width) were observed at 350 °C when using both dissolution media (HNO3 and CH3COOH) to prepare the eggshell and struvite solutions where NH4OH was used as the pH buffer. The Ca/P (atomic%) ratio of the samples produced ranged between 1.3 and 1.7, indicating formation of non-stoichiometric HA and Mg-WH.
A single phase HA along with brucite [Mg(OH)2] was formed by simple ball milling of calcined eggshell powder and struvite , while biphasic CaPs (i.e. HA and Mg-WH) alongside MgO was produced at higher sintering temperatures (i.e. 900 and 1150 oC) using either raw or calcined eggshells and struvite. The ranges of crystallite size of HA and Mg-WH were found to be 30.5 – 80.6 and 28.45 – 82.5 nm, respectively.
The CaPs microspheres were manufactured successfully from ball-milled raw/calcined eggshell-struvite of two granules size ranges (63 – 100 and 100 – 160 µm) via flame spheroidisation process. Single-phase of α-TCP was formed for the ball milled raw eggshell -struvite samples (smaller size granules), whereas biphasic CaPs (i.e. α-TCP and HA) was formed when calcined eggshell was used with struvite for ball milling. The Ca/P ratio of microspheres were found to be within the range of 1.5 – 1.65.
CaP-Ch composites (i.e. scaffolds and film) were fabricated successfully via a freeze drying and casting method. Resonance acoustic mixing (RAM), a newly developed mixing method, was used to prepare a homogeneous composite suspension prior to freeze drying. Crosslinking of the scaffolds was also performed by using genipin, a naturally-derived crosslinker. Morphological, structural, compositional, swelling properties, mass loss and mechanical properties were evaluated for the composites. Interconnected porous structure was observed for all the scaffolds. However, the reduction in porous structure and mass loss and no significant change in mechanical properties for the cross-linked scaffolds compared to bare scaffolds was observed. This was due to the partial crosslinking of scaffolds by genipin, confirmed via FTIR. Films of the same formulations (except using CaPs derived via hydrothermal synthesis) were also fabricated using casting method and their properties were evaluated. The films containing commercial HA exhibited highly interconnected porous structure. However, a film comprising ball-milled CaPs showed a moderate range of swelling percentage (211 %) with highest mass loss (about 34 % after 96 hrs) in comparison to other formulations.
This study shows that use of eggshells and struvite offer a promising resource as potential sustainable precursors, along with acetic acid as sustainable dissolution media to produce calcium phosphates with varying morphologies, microspheres and their composites with chitosan for biomedical applications.
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